The present invention relates to a machine tool having at least two tables on which at least one workpiece is mounted.
In a conventional machine tool such as a machining center or a drilling center, a palette and a palette changer is installed in order to minimize period for exchanging the workpieces to thereby enhance workability or productivity as described in Japanese Utility Model Application Kokai No. 63-57035. More specifically, according to the conventional machine tool, a stationary machining table is provided, and a plurality of palettes each mounting a workpiece thereon can be moved between the stationary table and the palette changer by palette transferring mechanism. Further, a spindle head is movable in three directions, i.e.,X, Y and Z directions extending perpendicular to one another for accessing and machining the workpiece. With such structure, no operational suspensions occur attendant to attachment and detachment of the workpiece to and from the palette.
Such conventional arrangement, however incurs the following disadvantages:
(a) It would be rather difficult to provide the spindle head having high rigidity, since the spindle head is movable in three directions.
(b) High machining accuracy may not be obtainable, since machining error may be conceivable due to three dimensional motions of the spindle head and due to inaccurate fixing of the palette to the stationary machining table. That is the error may be accumulated by the three dimensional motions of the spindle head and inaccurate positioning of the palette with respect to the stationary table.
(c) Various mechanical components are required for transferring the palette and for positioning the palette at a given position of the stationary machining table.
(d) Required are large weights of movable components such as the spindle head, a column for supporting the spindle head and a column base for supporting the column in order to obtain high rigidity capable of performing three dimensional movements of the spindle head. As a result, entire machine tool becomes bulky and moving speed of the spindle head may be lowered.
(e) In connection with item (d), such heavy movable components may be shifted at one location of the machine tool in actual machining. In this case, imbalance in weight may result.
(f) In connection with item (d), entire size of the machine tool becomes large in comparison with the size of the workpiece on the palette. As a result, a spacial problem may occur for installation of the machine tool in a limited space of a factory.
(g) During machining to one workpiece on one palette, another palette mounting another workpiece thereon is introduced into the stationary machining table and is clamped thereto. Further, during machining to the one workpiece on one palette, another palette may be unclamped from the table. When the palette stops moving, or when the palette is clamped or unclamped, or when the palette starts moving, impact force may be propagated to another palette undergoing machining. As a result, machining accuracy may be lowered with respect to the workpiece subjected to machining.
(h) The spindle head is movable in one horizontal direction so as to change machining spots, which horizontal direction is directed perpendicular to the travelling direction of the palette. In this case, the moving area in one machining spot may be decreased in accordance with the increase in numbers of the palette. As a result, it would be impossible to perform machining to a single large scale workpiece.